i play the GC 39 and after a DV in Warsaw (i choose South) i fought in Greece to help the overwhelmed Italians. After this, Pancer Corps bring me to the Eastfront (Smolensk and others)
I think there is something wrong?!
Most likely some sort of bug, but without actual save files or anything of that nature to look at, its impossible to determine what, if anything, went wrong.
I would recommend reverting your campaign to a older save point so you can make the decision for Warsaw South again (or replay the campaign if worst comes to worst). I highly doubt it will send you to Greece again. If it does, however, we would be very interested to see save files and campaign files to help figure out where the breakage is occurring.